1. Field of the Invention
The present invention relates to a fan-in/fan-out device for multicore fiber.
2. Description of the Related Art
A multicore fiber has been studied to overcome rapid increase of transmission capacity of the recent years and the transmission capacity limit per one optical fiber.
In order to utilize the multicore fiber, a fan-in/fan-out device is indispensable as an input/output device which connects each core of the multicore fiber and an external optical fiber.
FIG. 11 shows an input/output device using one example of a fan-in/fan-out device. The input/output device shown here includes a multicore fiber 1, fan-in/fan-out devices 20, each of which is connected to each end of the multicore fiber 1, and external optical fibers 3 connected to the devices 20.
The device 20 includes a plurality of single-core fibers 22 and a holding portion 8 which holds the single-core fibers 22.
Each of the single-core fibers 22 include a large diameter portion 24 and an elongated portion 27 which extends from the large diameter portion 24.
The elongated portion 27 includes a diameter-reduced portion 25, a diameter of which is reduced from a diameter of the large diameter portion 24 and which extends from the large diameter portion 24, and a small diameter portion 26 which extends from the diameter-reduced portion 25. In the diameter-reduced portion 25, the core diameter becomes smaller towards the extending direction.
The elongated portion 27 can be formed by heating a portion of the single-core fiber 22 and fusing and elongating the portion.
The large diameter portion 24 is connected to the external optical fiber 3 at the splice point C1. The small diameter portion 26 is connected to each core of the multicore fiber 1 at the splice point C2.
The device 20 is capable of injecting light into the core of the multicore fiber 1 from the external optical fiber 3 via the single-core fiber 22 or injecting light into the external optical fiber 3 from the core of the multicore fiber 1 via the single-core fiber 22.
In the single-core fiber 22, when the core diameter becomes small, the light confinement ability becomes low and the mode field diameter becomes large. Therefore, mismatch of each mode field diameter occurs at the splice point of the multicore fiber 1 and the device 20, and the splice loss easily increases.
In order to solve the problems, a core with a double structure is proposed (see “PROFA Pitch Reducing Optical Fiber Array,” searched on Jun. 6, 2013, http://www.chiralphotonics.com/Web/profa.html).
The device using the core with a double structure is described with reference to FIG. 11 in addition to FIGS. 12-13B.
FIG. 12 is a schematic view showing a structure of the single-core fiber 22 of the device 20. FIG. 13A is a graph showing a refractive index distribution of a non-elongated single-core fiber and an electromagnetic field distribution of light. FIG. 13B is a graph showing a refractive index distribution of an elongated single-core fiber and an electromagnetic field distribution of light.
As shown in FIGS. 11 and 12, the device 20 includes single-core fibers 22, each of which has a double-structured core 22a and a cladding 22b which covers an outer surface of the core 22a. The core 22a is configured by a center portion 22a1 with high refractive index and an outer periphery portion 22a2 with low refractive index that covers a surrounding of the center portion 22a1.
As shown in FIG. 13A, regarding the non-elongated single-core fiber 22, since the diameter of the center portion 22a1 is large, a strongly-localized mode exists at the center portion 22a1.
On the other hand, as shown in FIG. 13B, regarding the elongated single-core fiber 22, a mode which is transmitted through the center portion 22a1 does not exists since the diameter of the center portion 22a1 is gradually reduced. However, the light which is not confined in the center portion 22a1 is also transmitted as a mode which exists within the outer periphery portion 22a2. Therefore, based on a device having such structure, it is easy to design the device such that the variation of the mode field diameter after the elongation is reduced.
In the device 20, since the mode field diameter increases when the light is transmitted from the center portion 22a1 to the outer periphery portion 22a2, the electromagnetic field overlaps with an electromagnetic field of other adjacent optical fibers 22, and a large amount of crosstalk occurs.
If an optical fiber having a refractive index distribution with a single peak is used, the above-described phenomena do not appear; therefore, the overlap of the electromagnetic field can be prevented and crosstalk can be reduced.
However, since the mode field diameter greatly changes due to the core diameter changes at the diameter-reduced portion, it is difficult to reduce the difference of the mode field diameter with respect to an optical fiber to be connected (i.e., the external optical fiber 3 or the multicore fiber 1) at both splice points C1 and C2. Therefore, the splice loss becomes greater at either the splice point C1 or the splice point C2, and the splice loss increases as a whole.